Image forming apparatus

ABSTRACT

In an image forming apparatus which forms a color image by superposing cyan, magenta, yellow and black toners on a recording medium, a circuit to limit image signals is provided immediately before a gamma correction circuit which corrects an output characteristic inherent in the apparatus so that the sum total of the signal levels of the image signals of the respective colors does not exceeds a predetermined value. Thus, even if a user arbitrarily sets color profile data for color management, a toner spread amount can be controlled not to exceed the predetermined value.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an image forming method which forms a full-color image by using multicolored coloring materials.

[0003] 2. Related Background Art

[0004] In a conventional full-color image forming apparatus, it is impossible to exhaustlessly spread coloring materials on a medium such as a paper or the like. That is, if it intends to superpose the coloring materials of which the total amount exceeds a certain saturation point, a situation which damages the image forming apparatus itself might occur.

[0005] For example, in a case where the full-color image forming apparatus adopts a electrophotographic method, if toner of which the amount exceeds an allowable amount is spread on the same area of a photosensitive drum, the spread toner comes to be poorly transferred to a medium, whereby a treatment of the residual toner on the drum after such a toner transfer operation ended comes to be a problem. For example, in this case, the unexpected residual toner is supplied to cleaning parts as a matter of unpleasant necessity. Therefore, in the worst case, there is a fear that the cleaning parts are demanded.

[0006] Moreover, in a so-called cleaning-less system which has been designed environment-friendly so that waste toner is not exhausted outside the image forming apparatus as much as possible, if toner of which the amount exceeds an expected amount is spread on a photosensitive drum, the residual toner on the drum still exists after the toner transfer operation ended. Thus, the residual toner blurs an electrification system, and thus affects an electrification capacity of the system. In addition, there is a high possibility that the residual toner causes undesirable color mixture in an another-color image forming station, whereby an undesirable situation that an image cannot be output according to the exact colors occurs.

[0007] Moreover, for example, even if a large amount of toner can be spread on the same area on a medium, in a thermocompression fixing method by a fixing roller which is frequently adopted in the image forming apparatus of the electrophotographic method, the image forming apparatus lacks an amount of heat because the amount of spread toner is so large, whereby poor fixing occurs. On the other hand, even if the toner can be fixed or adhered to the medium for the time being, there is a problem that a large amount of fused toner is generated, whereby the fused toner cannot be completely fixed to the medium, and offset by the fused toner occurs on the fixing roller. Moreover, it becomes difficult for the toner to be separated from the fixing roller, whereby the medium is accidentally wound around the fixing roller.

[0008] Such situations as above are equal to destruction of machine. Therefore, if such a trouble occurs on the user side, since a user cannot cope with it alone, for example, he has to bring the broken machine to a service center or call a serviceman to repair the broken machine.

[0009] Moreover, even in an ink-jet printer, if an excessive ink is discharged or emitted to a medium, a problem of bleeding (a phenomenon that the ink flows out on a medium before it gets dry) occurs. Besides, there is a problem that an unexpected unusual situation that the ink penetrates through the medium and reaches the back side thereof occurs.

[0010] Thus, in this case, when a generation amount of a black ink is determined, conventionally a UCR (under color removal) process is performed to reduce the maximum signal sum total for cyan, magenta, yellow and black inks. Here, it should be noted that, if the maximum value of one color ink is assumed to be 100%, the maximum signal sum total is frequently defined by numeric values of 270%, 300% and the like.

[0011] Moreover, a color balance is changed so that the amount of spread toner does not increase too much.

[0012]FIG. 2 is a diagram showing an image data processing flow of the conventional full-color image forming apparatus. Incidentally, there are various types of image data which are dependent on intended purposes and use applications, that is, there are RGB-system image data which are representative of sRGB-system image data based on the three primary colors of a CRT monitor, and CMY-system mage data which are mainstream in the printing world. Here, it should be noted that symbols R, G, B, C, M, Y and K respectively denote red, green, blue, cyan, magenta, yellow and black.

[0013] Even when the image data of which system are used, the image data to be handled are finally converted into color signals which are optimum for coloring materials (C, M, Y and K coloring materials in most cases) to be used in an image output apparatus, and a color matching process is performed based on the obtained color signals, whereby an output material of beautiful and satisfactory colors can be obtained.

[0014] Such a color conversion is performed by CMM's (color management modules) 121 and 122. In order to secure color accuracy of the image output apparatus with a higher precision, the CMM's 121 and 122 have conversion characteristics as profiles 128 and 129 respectively, whereby a user can customize the CMM in accordance with a characteristic of the image output apparatus by appropriately changing the conversion characteristic.

[0015] As a means for achieving high-precision color matching, there is an ICC profile which has been proposed by ICC (International Color Consortium). Recently, such a technique begins to be generally used for a proof of color printing (in the proof, an output material substantially equivalent to the material to be finally printed is output and checked before the final printing is performed), and has been available as Color Profile Maker™ and Color Profile Builder™ in the form of software for a tool to accurately perform the color matching.

[0016] The C, M, Y and K signals which have been subjected to the color matching are then input to a digital spatial filter circuit 123. In the digital spatial filter circuit 123, the sharpness of the input signals are appropriately adjusted. After then, the C, M, Y and K signals are input to a gradation adjustment circuit 124. In the gradation adjustment circuit 124, the gradation of the colors represented by the input signals can be appropriately adjusted according to a user's inclination. Then, the processed signals are further input to a gamma LUT (look-up table) circuit 125. In the gamma LUT circuit 125, a gradation nonlinear characteristic of the image output apparatus is corrected on the basis of the input signals.

[0017] After then, the C, M, Y and K signals are input to a digital halftoning processing circuit 126. In the digital halftoning processing circuit 125, for example, the input signals are processed to increase multidither screen dots in a gradation reproduction method optimum for the image forming apparatus, or to increase the number of dots for unit area according as the gradation becomes denser as in an error diffusion method. Finally, the processed signals are input to a writing element driver circuit 127, whereby an image is formed on the basis of the input signals.

[0018] Here, it should be noted that the gradation adjustment circuit 124 is provided to perform the input/output conversion of the C, M, Y and K signals and adjust a color balance of these signals.

[0019] That is, in order to form a full-color image, each of the C, M, Y and K signals output from the digital spatial filter circuit 123 is plane-sequentially processed with the corresponding color ink through the gradation adjustment circuit 124, the gamma LUT circuit 125 and the digital halftoning processing circuit 126, and then the images of the four colors are multiplexed together.

[0020] However, even if the amount of spread toner is controlled in the UCR process and the color balance adjustment described as above, since the respective profiles 128 and 129 of the CMM's 121 and 122 can be set directly by the user, it is not likely to be able to correspond to the adjustment of the amount of spread toner according to the contents of the profile data of the CMM directly set by the user.

[0021] That is, in case of adjusting the color balance in the gradation adjustment circuit 124, when the color balance has been set so that the density becomes higher, if it is adjusted by the CMM to set the plural colors of C, M, Y and K to become simultaneously higher, the amount of spread toner might exceed an expected amount.

SUMMARY OF THE INVENTION

[0022] An object of the present invention is to provide image forming method and apparatus which can eliminate the above conventional inconvenience.

[0023] Another object of the present invention is to provide image forming method and apparatus which provide a processing circuit to limit the sum total of input color image signals so that it does not exceeds an arbitrary level and thus can significantly reduce a probability that an excessively spread coloring material damages the image forming apparatus itself.

[0024] Still another object of the present invention is to provide image forming method and apparatus which limit an amount of spread coloring materials immediately before data conversion is performed by using an LUT for correcting a signal conversion characteristic inherent in the image forming apparatus.

[0025] Other features of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIG. 1 is an image processing structural diagram for explaining an image forming apparatus according to the first embodiment of the present invention;

[0027]FIG. 2 is a diagram showing an image data processing flow of a conventional full-color image forming apparatus;

[0028]FIG. 3 is a diagram for explaining a concept of signal conversion in a coloring material spread amount limitation circuit;

[0029]FIGS. 4A and 4B are diagrams showing an example of a multidimensional LUT; and

[0030]FIG. 5 is an image processing structural diagram for explaining an image forming apparatus according to the second embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] Hereinafter, the embodiments of the present invention will be explained with reference to the attached drawings.

[0032] (First Embodiment)

[0033]FIG. 1 is an image processing structural diagram for explaining an image forming apparatus according to the first embodiment of the present invention. The image forming apparatus shown in FIG. 1 is different from the conventional image forming apparatus shown in FIG. 2 in the point that a coloring material spread amount limitation circuit 11 is provided between a gradation adjustment circuit 24 and a gamma LUT circuit 25. Therefore, it should be noted that the structure of the image forming apparatus in the present embodiment is the same as the structure of the conventional image forming apparatus except for the coloring material spread amount limitation circuit 11. That is, color conversion is performed by CMM's 21 and 22. Moreover, in order to secure color accuracy of the image forming apparatus with a higher precision, the CMM's 21 and 22 have conversion characteristics as profiles 28 and 29 respectively, whereby a user can customize the CMM in accordance with a characteristic of an image output apparatus by appropriately changing the conversion characteristic.

[0034] Thus, C, M, Y and K signals which have been subjected to color matching are input to a digital spatial filter circuit 23. In the digital spatial filter circuit 23, the sharpness of the input signals are appropriately adjusted. After then, the C, M, Y and K signals are input to the gradation adjustment circuit 24, and further input to the gamma LUT circuit 25. In the gamma LUT circuit 25, a gradation nonlinear characteristic of the image output apparatus is corrected on the basis of the input signals.

[0035] After then, the C, M, Y and K signals are input to a digital halftoning processing circuit 26. In the digital halftoning processing circuit 125, such a gradation reproduction process as described above is performed. Finally, the processed signals are input to a writing element driver circuit 27, whereby an image is formed on the basis of the input signals.

[0036] More specifically, input R, G and B image data are converted into the device-dependent C, M, Y and K signals by the CMM 21 in accordance with color conversion information registered in the profile 28.

[0037] On the other hand, input C, M, Y and K image data are converted into the device-dependent C, M, Y and K signals by the CMM 22 in accordance with color conversion information registered in the profile 29. Then, the C, M, Y and K signals converted and generated by the CMM 21 or 22 are processed by the digital spatial filter circuit 23 to optimize the spatial frequency characteristics thereof. More specifically, the digital process in the digital spatial filter circuit 23 is to perform a matrix operation on the basis of information of a target pixel and its peripheral pixels.

[0038] Next, the gradation adjustment process is performed by the gradation adjustment circuit 24. More specifically, the gradation adjustment circuit 24 performs the gradation adjustment process, a color balance adjustment process, a masking process and the like in a method such as a method of determining an output signal by operating an input signal, a method of converting an input signal by using an LUT, and the like. Moreover, the coloring material spread amount limitation circuit 11 is provided between the gradation adjustment circuit 24 and the gamma LUT circuit 25.

[0039] Hereinafter, an operation of the coloring material spread amount limitation circuit 11 will be explained.

[0040]FIG. 3 is a diagram for explaining a concept of signal conversion in the coloring material spread amount limitation circuit 11 which is operated on the basis of a multidimensional LUT. Here, it should be noted that, since a four-dimensional phenomenon cannot be well shown in an explanation diagram, the operation of the multidimensional LUT will be explained on the basis of a case example that only C, M and Y three colors are used.

[0041] In FIG. 3, the three axes respectively indicate the signal scales of C, M and Y, and the point of 0% indicates the point where no coloring material is spread. Here, it should be noted that the scale of each axis is determined so that the maximum amount of spread coloring material of each color becomes 100%.

[0042] If it is assumed that, in a case where the image forming apparatus reproduces various colors as appropriately blending the coloring materials, an abnormal phenomenon happens when the coloring materials are spread 180% or more, it is necessary to limit the signals within the curve indicated by the arrows. In practice, the limitation is performed by combining also black.

[0043] Hereinafter, how to set the multidimensional LUT will be explained.

[0044] Incidentally, it is best for the multidimensional LUT to have all the combinations of the signals on the input side. However, since a memory capacity is limited in nature, rough lattice points are registered beforehand. Then, if a signal between the adjacent lattice points is input, an interpolation operation is performed to the input signal.

[0045]FIGS. 4A and 4B are diagrams showing an example of the multidimensional LUT.

[0046] More specifically, FIG. 4A shows input C, M, Y and K data strings, and FIG. 4B shows output data strings. Here, a case example of 180% limitation is shown in FIGS. 4A and 4B.

[0047] The input signal (0, 0, 0, 0) (the order of values in parentheses is C, M, Y and K) shown at the uppermost row of FIG. 4A is given as the signal (0, 0, 0, 0) shown at the uppermost row of FIG. 4B, and similarly the input signal (0, 0, 220, 220) is given as the signal (0, 0, 220, 220). Likewise, the input signal of which the coordinates point is between the above successive input signals is linearly interpolated to generate the output signal. For example, the input signal (0, 0, 110, 110) is converted into the output signal (0, 0, 110, 110). That is, since the relation between the input signal and the output signal is maintained in this case, also the interpolated value of the input signal between the previously registered successive lattice points is maintained as it is.

[0048] Furthermore, the input signal (0, 0, 255, 255) is converted into the output signal (0, 0, 230, 230), and the signal (0, 0, 240, 240) is linearly compressed by the interpolation into the signal (0, 0, 226, 226).

[0049] In the example shown in FIGS. 4A and 4B, the signal levels of yellow and black are set to be the same with each other. However, in a case where the signal level of each color is different from others (the signal level is 221 or more), the linear interpolation for each color is performed between the signal levels 220 to 230.

[0050] Moreover, in a case where the signal levels of three colors are not 0, the input signal (0, 130, 130, 130) is given as the output signal (0, 130, 130, 130) without change on the table. However, the input signal (0, 255, 255, 255) is converted into the output signal (0, 153, 153, 153), and the input signals between the input signals (0, 130, 130, 130) and (0, 255, 255, 255) are linearly interpolated. Moreover, in a case where the signal level of each color is different from others (the signal level is 130 or more), the linear interpolation for each color is performed between the signal levels 130 to 153.

[0051] Moreover, in a case where the signal levels of all four colors are not 0, the input signal (100, 100, 100, 100) is given as the output signal (100, 100, 100, 100) without change on the table. However, the input signal (255, 255, 255, 255) is converted into the output signal (115, 115, 115, 115), and the input signals between the input signals (100, 100, 100, 100) and (255, 255, 255, 255) are linearly interpolated. Moreover, in a case where the signal level of each color is different from others (the signal level is 100 or more), the linear interpolation for each color is performed between the signal levels 100 to 115.

[0052] To explain simply, FIGS. 4A and 4B merely show the representative points. However, it is needless to say that it is better in practice to set the parameters of the four-dimensional LUT in more detail so that the points to be linearly converted do not become discontinuous. Moreover, it is possible to change a limitation ratio according to the color if necessary.

[0053] The signal of which the signal level has been limited is corrected by the gamma LUT 25 which corrects the nonlinear gradation component inherent in the printer, the corrected signal is then subjected to the gradation process by the digital halftoning processing circuit 26 according to the gradation reproduction method suitable for the image forming apparatus, and the processed signal is finally supplied to the writing element driver circuit 27.

[0054] As above, according to the present embodiment, the process by the coloring material spread amount limitation circuit 11 is performed immediately before the process by the gamma LUT 25 which corrects the nonlinear gradation component inherent in the image forming apparatus is performed. Thus, even if any profile is used in the CMM, it is possible to precisely limit the amount of spread coloring material, whereby it is possible to significantly reduce the probability that the excessively spread coloring material damages the image forming apparatus itself.

[0055] (Second Embodiment)

[0056] It is well known that a limitation amount of coloring material is different according to a kind of medium on which an image is to be recorded by the coloring material. That is, when an image forming process in which an electrophotographic system using toner as a coloring material is adopted is performed, if a medium such as a thick paper is used, thermal energy is absorbed by the medium itself, whereby a coloring material fixing condition cannot be satisfied. Therefore, if it intends to fix (or adhere) the coloring material to the medium at the fixing speed same as that in case of fixing the coloring material to an ordinary-thickness medium, it is necessary to reduce an amount of toner. Alternatively, it is necessary to lower the fixing speed so as to assure sufficient thermal energy.

[0057] On one hand, even when an image forming process in which an ink-jet system for discharging ink to a recording medium is adopted is performed, since an ink acceptance amount of the media of which the ink absorption speed is high is different from that of the media of which the ink absorption speed is low, it is necessary to limit a signal level according to a kind of medium.

[0058]FIG. 5 is an image processing structural diagram for explaining an image forming apparatus according to the second embodiment of the present invention. In the present embodiment, if a user of the image forming apparatus selects a medium of which the material is suitable for an image to be formed on the selected medium, a CPU 53 which received media information 52 corresponding to the selected medium selects and sets profiles 28 and 29 which are optimum for the selected medium. Moreover, the CPU 53 selects a 4D-LUT (four-dimensional LUT) 51 corresponding to the selected medium, and sets a coloring material spread amount limitation circuit 11. Here, it should be noted that, since other circuit components shown in FIG. 5 are substantially the same as those of the same reference numerals shown in FIG. 1 of the first embodiment, the explanation thereof will be omitted.

[0059] As above, according to the present embodiment, each parameter is set according to the kind of medium. Thus, even if various kinds of media are used, it is possible to prevent that the image forming apparatus is damaged due to excessively spread ink, whereby it is possible to achieve the optimum image forming process.

[0060] (Third Embodiment)

[0061] There is a case where a user intends to suppress a coloring material consumption amount even if an image quality level is disregarded. For example, if the amount of remaining coloring material is decreasing while printing is being performed, the user might wish to give priority to completion of image output even if color reproduction degrades.

[0062] In such a case, in addition to a mode to use an ordinary amount of coloring material, a mode to use a less amount of coloring material is prepared beforehand in an image forming apparatus. Thus, if the user selects the mode to use the less amount of coloring material, a parameter which is to be ordinarily used in a coloring material spread amount limitation circuit is replaced by an extremely low parameter such as a spread amount 120% or the like, whereby it is possible to provide a so-called consumption saving mode. 

What is claimed is:
 1. An image forming method in an image forming apparatus which forms a color image by adhering plural coloring materials to a recording medium, said method comprising: an input step of inputting an image signal; a color conversion step of performing a color conversion process to the input image signal, on the basis of profile data; a gradation conversion step of performing a gradation conversion process to the image signal subjected to the color conversion process in said color conversion step; a correction step of correcting an output characteristic inherent in the image forming apparatus; and a limitation step of limiting the sum total of signal levels of the image signals of respective color components, in regard to the image signals subjected to the gradation conversion process in said gradation conversion step.
 2. An image forming method according to claim 1, wherein said limitation step limits the sum total of the signal levels of the image signals of the respective color components, on the basis of a multidimensional look-up table on which the signal levels of the image signals of the plural color components are used as inputs.
 3. An image forming method according to claim 2, wherein said limitation step obtains a limitation value for the signal level of the image signal of each color component by an interpolation operation, if the signal level of the input image signal of the color component does not exist on the multidimensional look-up table.
 4. An image forming method according to claim 1, wherein said limitation step changes a limitation value of the sum total of the signal levels of the image signals of the respective color components, in accordance with a kind of recording medium on which the image should be formed.
 5. An image forming method according to claim 1, wherein said limitation step sets a limiting value of the sum total of the signal levels of the image signals of the respective color components to be smaller than the ordinary, if a mode to reduce a use amount of the coloring materials less than the ordinary.
 6. An image forming apparatus which forms a color image by adhering plural coloring materials to a recording medium, comprising: an input unit arranged to input an image signal; a color conversion unit arranged to perform a color conversion process to the input image signal, on the basis of profile data; a gradation conversion unit arranged to perform a gradation conversion process to the image signal subjected to the color conversion process by said color conversion unit; a gamma correction unit arranged to correct an output characteristic inherent in said image forming apparatus; and a limitation unit arranged to limit the sum total of signal levels of the image signals of respective color components, in regard to the image signals subjected to the gradation conversion process by said gradation conversion unit.
 7. An image forming apparatus according to claim 6, wherein said limitation unit includes a multidimensional look-up table which outputs, if the signals having the signal levels of the plural color components are input, the signal in which the sum total of the signal levels of the image signals of the respective color components has been limited.
 8. An image forming apparatus according to claim 7, wherein said limitation unit obtains a limitation value for the signal level of the image signal of each color component by an interpolation operation, if the signal level of the input image signal of the color component does not exist on said multidimensional look-up table. 